The present invention relates to a method for carrying out an automatic braking operation for a motor-vehicle brake system with an anti-lock system, and more particularly, to a method used in a system with a manually operable brake actuating device, the position of which determines the brake pressure during non-automatic normal braking, and with an anti-lock system (ABS) which detects reaching of the lock-up limit of a wheel and thereupon controls the brake pressure for at least this wheel. This method comprises steps of controlling, via the automatic braking operation, the brake pressure of at least one wheel not subjected to anti-lock system control so as to increase the brake pressure, and using the reaching of the lock-up limit of at least one wheel as the trigger criterion.
German Patent Application P 43 38 065.4, which is not a prior art publication, describes a method in which, in contrast to the method known from German Patent DE 40 28 290 C1 and Offenlegungsschrift DE 41 02 496 A1, in which the exceeding of a predetermined threshold value by the speed of actuation of the brake pedal is used as the trigger criterion for the activation of the automatic braking operation, does not require pedal-travel or pedal-pressure sensors. Instead, use is made of the information on the incipient locking up of at least one wheel, which information is detected and used in any case by the anti-lock system. This represents a useful trigger criterion because the tendency of the wheels to lock up occurs precisely in the critical driving situations in which the wheels not yet subjected to ABS control should likewise be subjected to an automatic braking action.
The automatic braking operation ensures that the wheels which are not yet locked up, because of differences in the skid-resistance properties of the roadway, in vehicle-weight distribution and/or braking-force distribution for example, and are therefore not being subjected to ABS control can be braked in an optimum manner. In order to achieve this optimum braking, the above methods envisage that, for the purpose of carrying out the automatic braking operation for the wheels not subjected to ABS control, the brake pressure should be set automatically to a higher value than would correspond to the instantaneous brake-pedal position. This is achieved by virtue of the fact that, at the beginning of the automatic braking operation, the brake lines leading to the wheels to be supplied with the increased brake pressure are decoupled from the master brake cylinder pressure, which is influenced by the brake pedal, and connected to an additional pressure source. For this purpose, use can, for example, be made of a pressure source already present for carrying out traction control (ASR).
This measure of using an additional pressure source to obtain increased brake pressure results in a so-called hardening of the brake pedal, making it impossible for the driver to gain reliable information on the braking effect at the wheels not subjected to ABS control during the automatic braking operation from the position of the brake pedal and its resistance to further actuation. This is the kind of information which the driver would be accustomed to obtaining during normal braking below the ABS lock-up limit of the wheels. In addition, the use of the additional pressure source results in a displacement of the pedal travel/brake pressure characteristic.
An object of the present invention is to provide a method for carrying out an automatic braking operation so that a vehicle can be braked in as short as possible a braking distance in critical driving situations while as far as possible maintaining the braking information available to the driver.
The foregoing object has been achieved in accordance with the present invention by providing that, during the automatic braking operation, the wheel not subjected to ABS control, of which there is at least one, and the mechanical brake-actuating device such are acted upon that the increased brake pressure for the at least one wheel not subjected to ABS control, does not exceed the brake-pressure value represented by the respective instantaneous position of the mechanical brake-actuating device.
As a trigger criterion for the automatic braking operation, use is made of the detection of the locking up of at least one wheel by the anti-lock system, with the development that the activation of the automatic braking operation takes place as soon as both wheels of one axle reach the lock-up limit and/or once the lock-up limit or the activation of the ABS is maintained for a certain time period. This approach eliminates the need for an additional sensor system for the detection of the demand for high braking forces in critical driving situations.
The brake pressure supply to each of the wheels at which locking up is detected and, more generally, to the wheels on the same axle as such wheels is controlled in the usual manner by the ABS during the automatic braking operation. In virtually all cases, the wheels on different axles lock up at different times due to differences in axle-load and/or braking-force distribution. However, as soon as the ABS control partially comes into effect, the driver feels a corresponding braking effect and therefore instinctively slackens off in his further depression of the brake pedal.
The purpose of the automatic braking operation is to provide better braking of the wheels which are, at least initially, not subjected to ABS control than is normally achieved solely by the action of the driver on the brake pedal. This is achieved by producing a certain brake pressure to be reached more rapidly and, since the driver frequently does not brake hard enough, generally also by setting a higher brake pressure than that demanded by the driver.
In order to provide the driver at all times, even during the automatic braking operation, with a brake-pedal feel which represents this increased brake pressure, or at least does not give him the impression of a lower brake pressure than that set, the brake-pressure increase takes place, according to the present invention, in correlation with the position of the brake-pedal-actuating device. The braking information which the driver continues to receive as a result furthermore makes it easier for him or her to take over control of the brakes again in a suitable manner when the automatic braking operation has been aborted on detection of an abort criterion, for which purpose one of the criteria known from the prior art cited at the outset, for example the return of the brake pedal beyond a specified position, can be used.
Not only the wheel not subjected to ABS control but, furthermore, the mechanical brake-actuating device are acted upon so that the latter yields to the demand for actuation by the driver at every point of the automatic braking operation, which requires little expenditure of force for the purpose, at least to the extent that its instantaneous position always represents a brake-pressure value which is at least as large as the set increased brake pressure at the wheel not subjected to ABS control.
The brake-pressure increase is preferably accomplished by appropriate control of a conventional brake servo unit with a brake pedal as a brake-actuating device. The setting of a higher brake-pressure value during the automatic braking operation than would correspond to the brake-pedal position during normal braking operations can be avoided by using, for example, an appropriate brake control unit to produce the increased brake pressure via a pressure source, with the brake fluid being taken from a master brake cylinder adjoining the brake pedal, and the brake pedal thereby being automatically adjusted accordingly at the same time. With such an implementation, the use of a further pressure source, from a traction control system for example, for increasing the brake pressure during the automatic braking operation is possible but not essential, and the method is thus suitable also for brake systems which have only an anti-lock system but not a traction control system and additional brake-pressure sources provided for the latter.
According to another aspect of the method of the present invention the brake pressure is increased in a variable manner to match the vehicle deceleration already achieved in each particular case, it being possible for the increased brake pressure to be adjusted to different values suitable in each particular case in the course of the automatic braking operation. Such a plausibility check of the deceleration achieved is advantageous particularly in those cases where the ABS control system responds at one wheel not because of a high overall braking-force demand but due to a locally low friction coefficient of the roadway, e.g. in the case of an icy patch travelled over by just one or two wheels, since unintentionally great and abrupt vehicle deceleration which might otherwise occur in these special situations due, for example, to a maximum brake-pressure increase is avoided by this matching of the increased brake-pressure value to the instantaneous deceleration present at a particular moment.